Circuit breakers have been designed to prevent the current through an AC line from exceeding a predetermined energy level to both prevent fires and to prevent damage to loads that are disposed on the lines. Typically, circuit breakers have utilized a thermal switch which, when reaching a certain temperature due to excessive current passing therethrough, will cause the breaker to trip.
There are a number of disadvantages to present circuit breakers, the primary one being the manner in which they make a trip determination. This trip determination is typically made based upon a predetermined amount of current passing through a thermally sensitive element, whose characteristics change at certain temperatures. These characteristics will cause a mechanical trip to occur at temperatures above a predetermined threshold. However, these type of breakers have little sensitivity to the type of load and merely react to the RMS current. For example, some loads require a large amount of inrush current when they start up. This can potentially trip a breaker unnecessarily. Another disadvantage is that with respect to "arcing," wherein the line has a small gap disposed therein, the current therethrough being intermittent but on the average less than necessary to trip the breaker. However, this arcing can result in sparks that can potentially cause a fire.
With respect to inrush current, this refers to the amount of current that is required whenever a line has voltage initially applied thereto. This inrush current differs from load type to load type. The decay or recovery of the startup load is another variable. Some loads, such as those associated with incandescent lighting, recovers in one to two half-cycles (8 to 16 milliseconds). Other loads, such as vacuum cleaners, have an exponential decay lasting 10 to 40 half-cycles (80 to 300 milliseconds), while motors which start under load, such as those used with air compressors, spas, and swimming pool pumps tend to maintain constant high current for several cycles, 10 to 20, then rapidly decay to their normal operating current.
Another condition that must be taken into account is the condition when a motor electrically or mechanically stalls during startup. In the case of mechanical stalling, the motor typically maintains its maximum startup current until the motor burns up and/or the breaker trips. Electrical stalling differs in that the motor is starved for starting current and is unable to develop enough speed to counter the EMF to achieve normal operating current. As the current is still quite high in this condition, this tends to damage the motor by overheating before the circuit breaker trips. Several things can cause electrical stalling: brownout (low line voltage), the wire being too small for the distance involved, connection contact resistance (wire nuts, contacts), and for multi-phase breakers where one leg is defective or tripped. All these electrical stalling conditions reduce the operating voltage to the motor causing a loss of current necessary to produce starting torque.
Even after the motor has successfully reached a startup condition without tripping the breaker, there is still the consideration of the motor load current surging during usage. These surge currents may range from low operating currents to maximum startup currents. Motor surging is often due to alternate loading and unloading of the motor, such as hydraulic resonance of water and entrapped air in a water pump system or an elastic coupling driving an inertial load in a mechanical arrangement.
The following table illustrates a typical residential circuit breaker trip time versus constant overload.
TABLE 1 ______________________________________ Typical Residential Circuit Breaker Trip Times vs Constant Overload % Of Breaker Circuit Breaker Rating 20 Amp Rating 30 Amp Rating Trip Time ______________________________________ 100% 20 Amps 30 Amps No Trip 200% 40 Amps 60 Amps 50 Sec 300% 60 Amps 90 Amps 12 Sec 500% 100 Amps 150 Amps 3 Sec 1,000% 200 Amps 300 Amps 0.62 Sec 5,000% 1,000 Amps 1,500 Amps 0.01 Sec ______________________________________
One of the primary disadvantages, as noted above, is that a conventional circuit breaker does not account for the differing type of load or for the variations of loads during operation thereof It merely trips when the load current exceeds its rating for a specified time or fails to trip when it in actuality should due to arcing, etc. There is a class of circuit breakers, called magnetic, which will trip immediately when instantaneous currents exceed 8.times.10x of the breakers ratings. However, again these breakers do not account for differing type of loads and also are susceptible to false tripping because of their speed. Although the false tripping of the circuit breaker due to high inrush currents does protect the system, it is bad for the breaker, since the tripping of a breaker in and of itself can cause the breaker to fail. Present-day circuit breakers are typically rated for only one trip. The manufacturer recommends replacement of the breaker after each tripping. This, in part, is due to the fact that the breaker trips at any current level and, at high current levels, this can cause damage due to heavy arcing across the contacts.